Although wind gusts and air turbulence have long been known to present hazards to aircraft during takeoff and landing, attention has more recently been directed to rapid changes in wind velocity known as wind shear. One particularly hazardous form of wind shear can occur during thunderstorm activity when cool upper air flows downward in a column so as to create a pattern of radially outflowing air when it strikes the ground. This phenomenon is known as a microburst. When a low altitude aircraft passes through this pattern, as might occur during takeoff or landing, it successively encounters a head wind, a down draft, and a tail wind. The abrupt switch from head wind to tail wind causes the plane to lose air speed, and thus lift, which may cause it to crash before effective corrective action can be taken.
At major airports, high powered search radars or airport surveillance radars (ASR) are used for detecting the location of aircraft, and special Terminal Doppler Weather Radars (TDWR) are used for the purpose of detecting the presence of dangerous wind patterns, in the rain. Nearly all commercial airports require the use of an ASR for air traffic control. Unfortunately, however, the added weather radars are beyond the financial reach of a large number of smaller airports.
Accordingly, efforts have been made to provide added function to the existing search radar, ASR, so that it can also detect dangerous wind patterns. U.S. Pat. No. 4,649,398 describes how special signal processing may be added to an existing ASR to detect wind patterns such as a microburst event. The ASR antenna has two medium gain, vertically displaced, but overlapping beams in space. Electromagnetic waves are radiated by the lower beam, and the Doppler spectra of the received signals on both beams are processed so as to generate signals synthetically representing wind patterns occurring at low elevation. Various modes of operation are suggested: subtraction of Doppler spectra related to the upper beam from the Doppler spectra of the lower beam; determination of the ratio of the spectra and derivation of their mean value. This process can detect wet microburst events (those containing a considerable amount of rain) but is inhibited from detecting the equally dangerous dry microburst events (those containing very little rain) by the instabilities of the high power klystron transmitter. These instabilities limit the subclutter visibility of the radar system and thereby limit the ability to detect the small radar cross section of the moving dry microburst when it is in the presence of the much greater stationary ground clutter.
Solid-state transmitters are available that have the necessary stability to provide sufficient subclutter visibility for the detection of the dry microburst event, as well as having advantages in reliability, maintainability, safety, and support cost. Although practical solid-state transmitters can generate average power equal to or greater than the klystrons in use, solid-state transmitters tend to be limited by economic practicality to providing radiated waveforms that are lower in peak power, but longer in pulse duration. When a segment of this lower peak power, solid-state waveform is used in conjunction with the existing medium gain antennas, insufficient signal to noise ratio is obtained for achieving a high probability of detecting the very low level dry microburst event.
The subject invention provides a practical means for detecting the dry microburst event, while obtaining the several other benefits of using a solid-state transmitter.